Combustion synthesis processes involve highly exothermic reactions. These processes are well known to those skilled in the art and are disclosed, e.g., in U.S. Pat. Nos. 4,948,761, 4,957,885, 4,990,295, 5,006,290, 5,011,800, 5,030,600, 5,071,797, 5,143,668, 5,145,619, and the like. The disclosure of each of these patents is hereby incorporated by reference into this specification.
It is also known that exothermic reactions are difficult to obtain and control. Thus, for example, U.S. Pat. No. 2,886,454 of Todd discloses that " . . . certain exceedingly troublesome problems and limitations are encountered in any attempt to produce metal carbides of acceptable properties by exothermic procedures (column 1, lines 68-71). At column 2 of his patent, Todd discloses that "For such an exothermic reaction to be successful, it must produce a sufficiently high temperature to result in the formation of a carbide mass . . . . It has been found that most mixtures of metal oxides, aluminum, and carbon . . . fail completely to produce a carbide mass . . . . One difficulty in attempting to theorize as to whether a particular mixture will be operative is that the product is a complex of compounds that do not reveal ordinary valence relationships . . . ."
Many different attempts have been made to simultaneously form and densify an object by a combustion synthesis process, and these attempts usually have met with, at most, limited success.
Thus, e.g., in an article by Yoshinari Miyamoto et al. entitled "Simultaneous Synthesis and Densification of Ceramic Components under Gas Pressure by SHS" (appearing in "Combustion and Plasma Synthesis of High-Temperature Materials," Edited by Z. A. Munir et al., VCH Publishers, Inc., New York, N.Y., 1990, at pages 163-169), a combustion synthesis process is described in which gas pressure is isostaticaly applied to the reaction mixture prior to the time the mixture is ignited. Because the process of this patent applies pressure to the reaction mixture from every direction, it is difficult to obtain a shaped object. Furthermore, because it often takes a substantial period of time to prepare the sample for reaction, to preheat the gas which provides the isostatic pressure, to increase the pressure from such gas, and to cool the gas after the reaction, this process is often cumbersome, time consuming,and expensive.
Thus, e.g., in an article by Jerry C. LaSalvia et al. entitled "Densification of Reaction-Synthesized Titanium Carbide by High-Velocity Forging" (Journal of the American Ceramic Society, Volume 75, No. 3, March, 1992, pages 592-602), some of the problems with prior art combustion synthesis processes were illustrated. In the process of this publication, after a green compact had been ignited and converted into its final product, the resulting microstructure was collapsed by high velocity forging. LaSalvia disclosed that, in this process, there is a "time window" in which successful densification could be accomplished which is " . . . between 5 and 10 s after ignition" (see page 594). However, as is disclosed on pages 595-596 of this article, many of the densified products formed by this process evidenced a substantial number of microcracks.
Thus, e.g., at pages 229-237 of the same "Combustion and Plasma Synthesis . . . " publication, an article by S. D. Dunmead et al. (entitled "Combustion Synthesis in the Ti--C--Ni--Al System") discussed the drawbacks of the combustion synthesis reactions. At page 229 of this publication, the authors stated that "One of the major drawbacks in the combustion synthesis (SHS) of refractory materials is the highly porous nature of the products. This porosity is caused by three basic factors: (1) the molar volume change inherent in the reaction, (2) the porosity present in the unreacted sample, and (3), adsorbed gases." Dunmead et al. then describe a process in which, after ignition of the reaction mixture occurred, hydraulic rams were compressed to exert force upon the reaction mixture. However, the physical properties of the products produced by this process were far from ideal.
Dunmead was also issued U.S. Pat. No. 4,946,643, in which he and his copatentees described a process similar to that disclosed in his specification. It appears that the process of the Dunmead patent requires the use of a substantial amount of energy and is not readily suitable for preparing large shaped objects.
In the process of the Dunmead et al. patent, a compact of starting materials is charged to a die (such as the cylindrical graphite die depicted in FIG. 2), and the entire compact is heated up to its ignition temperature. In the embodiment illustrated in the Examples of the patent, the die assembly was heated to a temperature of from 933 to 1173 degrees Kelvin to cause ignition of the reaction mixture disposed within it.
In the process of the Dunmead et al. patent, a current was used which was sufficient to cause " . . . the die to rapidly heat up (approximately 1500 degrees K/minute) . . . " (see column 13 of the patent). Although the Dunmead et al. patent does not specify either the size of the die used or the electrical properties of the die assembly, it does disclose that the voltage used was only " . . . approximately 5 volts . . . ." Thus, it appears that a very large current would have to be used to raise the die/compact assembly 1,500 degrees Kelvin in one minute.
The use of such a very large current in the Dunmead et al. process requires a substantial amount of energy to be transmitted to die in a relatively short period of time. In addition to being inefficient, there is some indication in the literature that the transmission of such a large amount of electrical energy in a relatively short period of time may create electromagnetic radiation which, some believe, is harmful to human beings.
In the Dunmead et al. process, as soon as ignition of the compact occurred (as sensed by a thermocouple), " . . . the hydraulic rams were compressed to 20.7 MPa . . . and the electrical potential was turned off . . . " (see Example 1). Within about two minutes after ignition (see Column 5), the hydraulic rams had compressed the ignited compact.
It is an object of this invention to provide a combustion synthesis process in which ignition is is caused with only a minimal expenditure of energy.
It is another object of this invention to provide a combustion synthesis process which produces a densified product which is substantially stronger and less porous than comparable prior art processes.
It is yet another object of this invention to provide a combustion synthesis process which can readily, effectively, and economically produce relatively large shaped products with excellent properties.